Event:

In order to link perception and action the brain must have a spatially accurate representation of the visual world, so it can generate actions appropriate to the objects it perceives. The only way visual information enters the eye is through the retina, which moves constantly between brief fixations. The retinal location of targets for action is not useful for calculating movements to acquire those targets, because after a saccade the retinal location of a stable object changes. Two strategies have been postulated to calculate the accurate location of movement targets: Helmholtz suggested that the brain knows the command to move the eye, and therefore can use that motor command to update the sensory representation. This feedback from the motor system to the sensory system is now known as corollary discharge. Sherrington suggested that the brain can calculate accurate target location if it knows the position of the eye in the world, and the first step in this process is to know the position of the eye in the orbit. He postulated that this signal arose from oculomotor proprioceptors. The lateral intraparietal area (LIP) is a brain region important in choosing targets for saccadic eye movements, and solves the spatially accuracy problem using both Helmholtz’s and Sherrington’s strategies: a rapid, relatively inaccurate corollary discharge mechanism, and a slower, but more accurate proprioceptive mechanism. Neurons in LIP remap their receptive fields around the time of a saccade to compensate for the saccade (Helmholtz’s theory). The activity of LIP visual neurons is modulated by eye position (Sherrington’s theory). The eye position signal modulating the visual response in LIP arises from the representation of eye position in monkey Area 3a, the part of primary somatosensory cortex which represents the activity of muscle spindles. Inactivation of Area 3a impairs the monkey’s ability to generate saccades using long-term visuospatial memory. This importance of proprioception for long-term visuospatial memory is not limited to the saccadic system. Mice with lesions of the rodent equivalent of Area 3a also have a deficit in long-term visuospatial memory.